Fiber-reinforced polymer (FRP) composites have been used as high-strength, low-weight engineering materials to replace metals in aerospace structures such as primary structures of aircrafts. Important properties of such composite materials are high strength, stiffness and reduced weight.
Multiple layers of prepreg plies are commonly used to form structural composite parts that have a laminated structure. Delamination of such composite parts is an important failure mode. Delamination occurs when two layers de-bond from each other. Important design limiting factors include both the energy needed to initiate a delamination and the energy needed to propagate it.
A cured composite (e.g. prepreg layup) with improved resistance to delamination is one with improved Compression Strength After Impact (CAI) and fracture toughness (GIc and GIIc).
CAI measures the ability of a composite material to tolerate damage. In the test to measure CAI, the composite material is subject to an impact of a given energy and then loaded in compression. Damage area and dent depth are measured following the impact and prior to the compression test. During this test, the composite material is constrained to ensure that no elastic instability is taking place and the strength of the composite material is recorded.
Fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of a material for aerospace applications. Fracture toughness is a quantitative way of expressing a material's resistance to brittle fracture when a crack is present.
Fracture toughness may be quantified as strain energy release rate (Gc), which is the energy dissipated during fracture per unit of newly created fracture surface area. Gc includes GIc (Mode 1—opening mode) or GIIc (Mode II—in plane shear). The subscript “Ic” denotes Mode I crack opening, which is formed under a normal tensile stress perpendicular to the crack, and the subscript “IIc” denotes Mode II crack produced by a shear stress acting parallel to the plane of the crack and perpendicular to the crack front. The initiation and growth of a delamination is often determined by examining Mode I and Mode II fracture toughness.